Precision reference pressure supply system

ABSTRACT

A closed loop, selectively variable precision pressure reference system for testing pneumatic apparatus comprising all solid state digital control of a precision valve for establishing and maintaining the selected or desired reference pressure as a ratio between a positive pressure and a vacuum, the pressure ratio being sensed by a precision pressure sensor of the vibrating diaphragm type. The desired reference pressure is expressed as a binary word and the output of the pressure sensor is converted to a binary word, these two words being digitally compared and the difference therebetween being converted to an analog signal for adjusting the valve thereby closing the loop to maintain the selected pressure. The system is also capable of providing a precision pressure rate reference. The system automatically compensates for the inherent non-linear characteristics of the pressure sensor over its range of operation.

llnited States hatent n91 Klein et a1.

[ Feb. 26, 1974 PRECISION REFERENCE PRESSURE SUPPLY SYSTEM [75]Inventors: Charles D. Klem, Phoenix; Robert T. Riall, Scottsdale;Richard A. Wallace, Phoenix, all of Ariz.

J73] Assignee: Sperry Rand Corporation, New

York,N.Y. 22 Filedz Mar. 20, 1972 [21] Appl. No.: 235,954

[52] US. Cl. 137/4875, 137/624.1l

Primary Examiner-Alan Cohan Attorney, Agent, or Firm-Howard P. Terry Inurrrn PNZUMAHCS 5 7] ABSTRACT A closed loop, selectively variableprecision pressure reference system for testing pneumatic apparatuscomprising all solid state digital control of a precision valve forestablishing and maintaining the selected or desired reference pressureas a ratio between a positive pressure and a vacuum, the pressure ratiobeing sensed by a precision pressure sensor of the vibrating diaphragmtype. The desired reference pressure is expressed as a binary word andthe output of the pressure sensor is converted to a binary word, thesetwo words being digitally compared and the difference therebetween beingconverted to an analog signal for adjusting the valve thereby closingthe loop to maintain the selected pressure. The system is also capableof providing a precision pressure rate reference. The systemautomatically compensates for the inherent non-linear characteristics ofthe pressure sensor over its range of operation.

6 Claims, 2 Drawing Figures Ail RhPLY PAIENTEDFB261974 MANUAL s SELECTAND SELECT DISPLAY D IG ITAL R E PLY DIS PLAY SHEET 1 BF 2 MODE 5 sE LECT RATE E MAN-Aura. SELECT OFF 16 22 --p Q +PS G FREQ. RATE CONVERT. EBLE 23 Y STORAGE f REGISTER AND cou NTER DIGITAL .E suBTRAcToR- bBLANKING GATES (LSB's) CONTROL 0 l SPLAY 34 LOGIC DECODERS i DRIVERS ATE REPLY I FI-G.l0.

.- STORAGE DOWN 'R REGISTER COUNTER CONTROL 33 32 3 C PAIENIEII I$794,070

SHEETBUFZ NULL INDICAToR CoNvERTER 0 MULTIPLIER FREQ. 531 To DCCONVERTER 55 I I. Y

RATE

SLEW DETECTOR BUFFER R F C LOCK osCI I I AToR I 7 SOURCE SOURCE 35 1 OF0F PRESSURE VACUUM 30 RF 6 GATE 1 CouNTER 11 c I VIBRATING DIAPHRAGMSENSOR DRIVE T L sENsoR D ELECTRONICS 9 I LLI 0.

. I TO I NsT.

- UNDER F IG 1 b TEST PNEUMATICS PRECISION REFERENCE PRESSURE SUPPLYSYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates generally to pneumatic test equipment and moreparticularly to' apparatus for providing a precision pressure referencesource especially applicable as a laboratory tool and in one applicationto the testing of aircraft air data systems.

2. Description of the Prior Art Pressure reference sources have beenknown and used for many years as a laboratory and manufacturing testingtools as well as for testing and maintaining pneumatic systems such asair data systems for aircraft. In general these systems have relied onthe so called quartz tube system and the well known mercury manometricsystem. Both of these systems have objectionable characteristics. Forexample, both the quartz tube system and manometric system both involveservoed sensors and are thus subject to servo lag. Also, because of thephysical nature of these devices it is difficult to isolate them fromambient conditions and hence their stability leave something to bedesired. Their greatest drawback however is their inherent delicatenessand large size rendering them difficult, if not impossible, to be moved,i.e., they are permanent installations.

More recently, there has been proposed a pressure reference systemsimilar in many respects to the present system in which a valve means isused to establish a desired reference pressure as a predetermined ratiobetween a predetermined pressure source and a vacuum, which referencepressure is detected by a pressure sensor, converted into an electricsignal and fed back for comparison with a signal representing thedesired pressure the difference controlling the servo valve such as toreduce the difference to zero. This closed loop system is completelyanalog and depends on a phase comparison of the desired referencepressure sig nal and the feedback sensor signal and therefore is subjectto errors associated with analog servo loops.

SUMMARY OF THE INVENTION The present invention overcomes all of thedisadvantages of prior systems set forth above. It provides a pressurereference system that is more rugged and portable than the conventionalmanometric or quartz tube systems and is far less complex in its controlcircuitry and considerably less expensive. Furthermore, since thecontrolling error signal for the valve positioning loop is derived indigital format it is therefore more precise and rapid in operation.

Basically, the desired pressure reference is established either manuallyor automatically (if used with a digital automatic test equipment) as abit parallel binary word and stored in an input register. The actualpressure in the reference pressure chamber is detected by a pressuresensor, whose output is a signal having a period or frequency dependentupon such pressure. This pressure transducer is preferably of the typedisclosed in U.S. Pat. No. 3,456,508 entitled, Vibrating DiaphragmPressure Sensor Apparatus" issued July 22, 1969 in the name of R. H.Frische assigned to the same assignee as the present invention. Thissensor signal is converted to a 20 bit parallel binary word and storedin a further register. The outputs of these registers are .valve forcontrolling the pressure within the reference pressure chamber, anddetected by the pressure transducer, in a sense and to an amount toreduce said binary difference signal to zero. Since the valve requires aparticular position to maintain the reference pressure, an integral pathin the forward loop is required. Additional compensations are alsoprovided. The vibration frequency versus pressure characteristic of thepressure transducers is non-linear and therefore the analog error signalsupplied to the pressure control valve is gain controlled in a manner tocompensate for this non-linearity. Further, provision is made forrapidly slewing the reference pressure to the desired pressure when thedifference binary signal exceeds a predetermined value. This not onlyimproves system response but also reduces the requirements of thedigital-toanalog converter. In addition to providing a precise referencepressure, means are also provided for supplying a precise and selectablereference pressure rate, a feature particularly useful in testingaircraft air data equipment. Suitable digital data entry and replyindicators are provided for manual operation.

BRIEF DESCRIQTION OF THE DRAWINGS FIG. la together with lb is aschematic block diagram of the precision pneumatic pressure referencesystem embodying the teachings of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiment of thepresent invention constitutes pneumatic pressure reference equipmentwhich may be employed with larger automatic test equipment for airborneavionic equipment and is primarily designed for use in testing air datasubsystems of said airborne avionic equipment. In testing such air dataequipment a source of accurate pneumatic pressure is required and thisreference must be capable of supplying pressure (P corresponding tovarious altitudes, for example, ranging from sea level to over eightythousand feet and pressure rates (P corresponding to rates of climb ordive ranging from a few feet per second to several thousand feet persecond. Also, the same type .of pressure reference equipment may be usedto test airspeed and Mach number portions of the air data subsystems,the only basic difference being that the reference pressure P includesthe above P pressure, i.e., the systems are for all intents andpurposes, identical and hence the present specification will describeonly the P section.

Referring now to FIG. 1, the source of reference pneumatic pressure Pwill first be described. A pressure chamber 1 constitutes the pressurereference source which is connected to an output pneumatic connection 2adapted to be connected to the P (static) sensor input of the air datasystem being tested (not shown) as the output load. The pressure withinthe chamber or accumulator 1 is precisely regulated by a precisionpneumatic valve schematically illustrated at 3, which preferably may beof the well known flapper-valve type or alternatively of the type shownin U.S. Pat. No. 2,936,783 entitled, Electrohydraulic Servo ControlValve" issued May 17, 1960 in the name of R. A. Moffatt and assigned tothe same assignee as the present invention. This type of valve comprisescontrol element or flapper 4 the position of which relative to parts 5and 6 control the ratio of pneumatic pressures from two sources, asource of positive pressure 7 and a source of negative pressure orvacuum 8. By precisely controlling the valve position, any value ofpressure P may be established in the accumulator or reference chamber 1,the position of valve flapper 4 is electrically controlled by the signalsupplied to valve positioning coils 9 from valve drive amplifier 10 aswill be described below.

The reference pressure existing within chamber 1 is detected by means ofa pressure sensor or transducer 1 1. This sensor is preferably of thetype disclosed in the above-referred to US Pat. No. 3,456,508 and indeedit is the precision of this transducer which greatly contributes to theprecision of the overall system. As described in this patent, thetransducer comprises a diaphragm one side of which is subject to zero orvacuum pressure and the other side of which is subjected to the pressureP to be regulated. The principle of operation of the transducer 11 isbased on the natural vibration characteristic of a flat diaphragmsecured only at its periphery when subjected to varying pressure loads.Thus, as the diaphragm is deflected more or less by changes in thepressure applied to it, its stiffness changes and its mechanicalresonant frequency changes as a direct function of the applied pressure.The diaphragm is therefore constructed as part of an electromechanicalresonant system comprising the diaphragm and its drive/sense electronics12, the frequency of which is a direct function of the referencepressure P In essence therefore the output of drive/sense electronicscircuit 12 is a signal having a frequency that varies with the pressureP The advantage of this type of pressure signal is that it is readilyadaptable to digital techniques as will be described below.

In accordance with the teachings of the present invention, the referencepressure to be established and maintained within chamber 1 is selectablycontrolled either manually or automatically, through associatedautomatic test equipment, using a solid state digitally controlledclosed loop servo system in which the generation of the loop errorsignal is derived digitally thereby improving the accuracy and responseof known systems. If the system of the present invention is coupled withautomatic test equipment schematically illustrated at 15, the selectorswitch 16 is switched to AUTO and the reference pressure isautomatically programmed into input command gate 17 as a bit parallelbinary word via parallel data bus 18 in accordance with a preprogrammedair data equipment test sequence. The equipment under test is connectedto output port 2 and the programmed reference pressure applied thereto.In practice, of course, the performance of the equipment under test ismonitored by the automatic test equipment but this function is notpertinent to the present invention.

If the system of the present invention is to be operated manually,switch 16 is set to MAN and the reference pressure is manually selectedby means of manual select and select display unit 19. This unitcomprises a seven digit octal pushbutton array, each pushbutton (exceptone, of course) supplying three binary digits of a 20 bit binary wordrepresentation of the desired or commanded reference pressure P Suchdigital input displays are conventional and familiar to those skilled inthe art. The operator uses an arbitrary conversion code for entering thecommanded pressure. Selection of MAN on mode selector 16 also conditionsinput command gate 17 to transfer the selected 20 bit word to storageregister and counter 23. After setting up the commanded pressurereference, the operator depresses the command enter button 21 which, vialead 22, controls logic that transfers the stored command word inregister 23 to the digital subtractor 24. Storage register 23 is aconventional 20 bit latchable register well known in the art. Thepressure rate function associated with register and counter 23 will bedescribed later.

As stated, the vibrating diaphragm pressure sensor 11 and itsdrive/sensor electronics 12 supplies an output frequency as a precisefunction of the pressure within chamber 1. The pressure versus periodcharacteristic is as illustrated and is quite non-linear; for variationsin pressures at high pressures, e.g. sea level, the period variation issmall while at low pressures, e.g. high altitudes, the period variationis quite large. Means have been provided for completely compensating forthis characteristic as will be described.

The frequency output of the transducer electronics 12 is preciselyconvertible to a digital word. The sensor frequency is divided by ahexidecimal counter 30 that acts through gate 35 to enable and disableat equal time intervals a standard, stable high frequency clock 31 bymeans of suitable logic at the input of down counter 32 through inputline switches 36 which control count gain. When the gate generator 30enables the down counter 32, clock pulses are accumulated in a 20 bitstorage register 33 resulting in a stored digital word proportional tothe pressure P within accumulator 1. Upon a disabling signal fromcounter 30, indicating completion of the down count of counter 32, alatching signal is supplied via lead 34 to register 33 which transfersits contents to digital subtractor 24.

Subtractor 2 is a conventional digital subtractor operatingconventionally as a complement adder. Thus the digital signalrepresenting the commanded pressure is compared with the digital signalrepresenting the existing actual pressure in accumulator 1 therebyproviding at the output of subtractor 24 a 20 bit digital wordrepresenting the difference therebetween, i.e., a digital error word.This error word is supplied via 20 bit parallel bus 40 to adigital-to-analog converter/multiplier 41 and to a slew detector 42. Ingeneral, the D/A converter 41 is a l2 bit device, since it only has tolook at the first 12 bits of the error word. If the significant bits ofthe error word exceed twelve, the slew switch 42 recognizes this andsupplies a large plus or minus slew signal, depending on the sense ofthe error, to valve amplifier 43 through a suitable buffer amplifier 44.The D/A converter multiplier is a conventional 12 bit R-2R ladder drivenby transistor bit switches set by the l2 least significant bits of thedigital error signal from subtractor 24. For gain control purposes to beexplained below, the D/A converter 41 includes a multiplying function.This is accomplished by varying the reference voltage supplied to theR-2R ladder so that the output of the connector/multiplier will be theproduct of the reference voltage and the binary number set by the bitswitches. A suitable D/A converter adapted as described above to providethe multiple function is shown in Applicants assignees copendingapplication Ser.

No. 225,794, filed Feb. 14, 1972 in the name of Frank Kallio andentitled Bi Polar Digital to Analog Converter.

Thus, the input to amplifier 43 is either a fixed constant dc. voltagefrom slew detector 42 or a variable, gain controlled voltage from D/Aconverter 41 proportional to the 12 least significant bits of thedigital error signal from subtractor 24. The output of amplifier 43 isapplied to valve drive amplifier which energizes valve windings 9 tothereby position flapper valve 4 and .change the valve of pressure P inaccordance with the system error. This change in P will of course bereflected in a corresponding change in the output of transducer 11 andthus its digital word to subtractor 24 such as to reduce the erroroutput thereof to zero. Thus, the reference pressure P in chamber 1corresponds to that commanded by the manual P select panel 19 or by theATE 15. If the ATE mode is selected, the digital output of transducer 11is supplied via ATE reply bus 45 back to the ATE to thereby close theATE loop. If the manual mode is selected, the digital output oftransducer 11 is supplied via bus 45 to a 7 digit digital reply display46 through conventional decoder/driver circuits 47 whereby the numbersdisplayed on reply display 46 corresponds with the number displayed onthe select display 19. If desired, the least significant bits of the bitword may be blanked in order to eliminate any annoying dither of thelower of displayed digits. This is accomplished by means of a series ofblanking gates 48 responsive to the digital reply word from register 33and to an internal memory set by the command word on bus 20 from selectpanel 19. These gates control the display decoder/drivers associatedwith the lowest order numerals to be responsive to every second, fourth,eighth, etc. bit depending upon the pressure selected. This digit dithersuppression is most desirable when low (high altitude) pressures areselected due to the reduced sensitivity characteristic of the transducer11 at low pressures.

Since, the position of flapper valve 4 determines the reference pressureP this position must be accurately established and maintained. Thus, theoutput signal from D/A converter 41 is supplied to the amplifier 43which controls valve drive amplifier 10 to establish the requiredposition of the valve 4. This error signal output is also supplied to anintegrator 50 which supplies at its output the integral of the errorsignal which serves to maintain the required valve position as the errorsignal is reduced to zero. It will be noted however that if the slewdetector is supplying the error signal to the valve amplifier, theintegrator 50 is clamped via lead 51 responsive to the output of buffer44. The purpose of this is to prevent the integrator from having adestabilizing effect on the servo loop by preventing a rapid buildup ofthe integral signal by the large slewing signal. As soon as the errorreduces to the point where the D/A converter takes over, the integratoris unclamped and functions in its normal manner.

ln accordance with further features of the present invention, means areprovided for increasing the precession and response of the pressurereference system at extremes of pressures by controlling systemcharacteristics in accordance with the characteristics of the vibratingdiaphragm pressure transducer 11. As illustrated, at low pressures, thevibration period of the sensor varies rapidly with small changes inpressure while at high pressures, the vibration period varies relativelyslowly. Therefore, in order to accurately position the flapper valve 4in the high pressure region the gain of the valve positioning loop isincreased. This is accomplished by looking at the output of the pressuretransducer l1 and varying the output of the D/A converter/- multiplier41. as a function thereof.

Thus, the output of the transducer drive/sense electronics 12 issupplied via lead 52 to a frequency to d.c. converter 53 which suppliesa signal at its output 54 that increases in value as the vibrationperiod (or frequency) decreases. This signal provides the referencevoltage for the ladder of the D/A converter 41 and effectively performsa multiplication function to gain program the analog output of theconverter. A further gain adjustment may be provided by directlycontrolling the gain of amplifier 43 in accordance with the output 54 offrequency to d.c. converter 53. Also, in order to maintain stablecontrol of the valve, the gain of the integrator 50 is likewise variedas a function of the diagram period.

A further compensating means is provided for increasing the response ofthe flapper valve 4 to the error signal when in the high pressureregion. It will be recalled that when the slew detector 42 is effective,the integrator 53 is clamped. When operating in the high pressureregion, it is desired rapidly to bring the integrator output up towardits final valve when the slew signal is removed so as not to introduceany objectionable lag. This is accomplished by means of the frequency tod.c. converter 53, the output slew detector buffer 44, and a ratecircuit 55 coupled therebetween. When, under the control of the slewdetector 42, the pressure changes such as to reduce the digital error tothe threshold of the D/A converter, the slew detector is turned off. Itscollapsing d.c. signal is applied to rate circuit 55 and, ascharacteristic of such circuits, produces a substantial pulse of energy.If operating in the high pressure region, as detected by suitable logicresponsive to the frequency of the pressure transducer output, thispulse is applied to the integrator 50 to rapidly bring its output upnear its final value.

In accordance with a further feature of the present invention, means areprovided for supplying a precision reference pressure rate. This isparticularly useful in testing aircraft air data equipment for properrate of climb and drive operation and airspeed rate operation etc. Forthis purpose the system includes a reference pressure rate selectorwhich may include a knob and dial arrangement calibrated for example, infeet per second in the P embodiment herein illustrated. Setting of thisknob to the desired value of pressure rate provides a corresponding dc.voltage which is supplied to a conventional dc. to frequency converter61 which in turn supplies at its output a corresponding pulse trainhaving a repetition frequency corresponding to the desired pressurerate. This signal is applied to suitable logic gates having controlinputs for determining the sense of the selected pressure rate. For thispurpose a simple spring loaded center off double pole switch 62 suppliesa plus or minus d.c. signal to the sense gates to gate the pulse traininto register 23. The register 23 is conditioned in this rate modesimply to continuously count the pulses supplied to it to therebycontinuously increase or decrease the digital word supplied to thesubtractor 24 at the rate established by the pulse train frequency. Theerror signal is processed as above described to vary the valve 4position and hence vary the pressure in accumulator l at the selectedrate. The pressure transducer senses this changing pressure and theoutput frequency of its sensor 12 varies at a corresponding rate. Uponrelease of the switch 62 the system stabilizes out at whatever pressurewas established at switch release.

In the embodiment of the present invention described above, thevibrating diaphragm is temperature sensitive so that in order tomaintain accuracy of the reference pressure under varying embodiments,the sensor and its associated critical electronics are enclosed in aninsulated, temperature regulated enclo: sure. The operating pressurerange of the system is less than one to more than 45 inches of mercuryfor P and from less than 1 to 120 inches of mercury for P The digitaloutput resolution (using the 20 bit word) which varies with pressureranges from about 0.5 milli-inch Hg at the high pressure regions to 0.05milli-inch at the low pressure regions.

In order to achieve a more constant sensitivity in digital outputresolution the contents of the down counter 32 may be used inconjunction with a programmable control logic 37 and counter input linecontrol switches 36 which increase or decrease the counting rate of thedown counter 32 as a function of its own contents. This feature inconjunction with the use of a down counter permits the output digitalfunction from storage register 33 to be made approximately linear withoutput pressure. This technique can, be used to achieve a directpressure readout in inches of mercury or millibars by designing thecontrol logic 37 and input line control switch intervals to achieve goodsensor curve fit. Of course, using the digital techniques of the presentinvention, greater accuracies can be realized by increasing the bitlength of the digital word.

While the present invention has been described in a preferredembodiment, it is to be understood that the words which have been usedare words of description rather than limitation and that many changesmay be made in these constructions within the purview of the followingclaims without departing from the true scope and spirit of the inventionin its broader aspects.

We claim:

1. A reference pneumatic pressure supply system, the combinationcomprising a reference pressure chamber,

positionable valve means coupled with said chamber and a source of highpneumatic pressure and a source of low pneumatic pressure and adapted tocontrol the resultant of said pressures within said chamber,

means for providing a first multi-bit digital word corresponding to aselected pneumatic pressure reference,

pressure sensor means responsive to the pressure in said chamber forproviding a second multi-bitdigital word corresponding to the pneumaticpressure within said chamber,

means responsive to said digital words for providing a third multi-bitdigital word corresponding to the difference therebetween,

digital-to-analog converter means responsive to said difference word forproviding an analog error signal corresponding thereto,

means responsive to said analog error signal for positioning said valvemeans in a sense and to an amount to reduce said third digital word tozero,

slew detector means responsive to the most significant bits of saidthird multi-bit digital word for providing a constant analog voltageoutput corresponding in sense to the sense of said difference word, and

means for supplying said constant voltage to said valve positioningmeans whereby to slew the pressure existing in said reference pressurechamber rapidly toward said selected reference pressure.

2. The reference pressure supply system as set forth in claim 1 whereinsaid digital-to-analog converter means is responsive only to the leastsignificant bits of said digital difference word below the mostsignificant bits thereof to which said slew detector means isresponsive.

3. The reference pressure supply system as set forth in claim 1 whereinsaid valve positioning means further includes integrator meansresponsive to said analog error signal for maintaining said valveposition as said error signal is reduced to zero, said system furtherincluding means responsive to an output of said slew detector means forclamping said integrator.

4. The reference pressure supply means as set forth in claim 3 furtherincluding,

means responsive to the rate of change of the output of said slewdetector for supplying a signal pulse upon the output of said slewdetector going to zero and,

means for supplying said signal pulse to said integrator means torapidly increase the output of said integrator.

5. A reference pressure supply system comprising the combination of areference pressure chamber,

positionable valve means coupled with said chamber and a source of highpneumatic pressure and a source of low pneumatic pressure and adapted tocontrol the resultant of said pressures within said chamber,

means for providing a first multi-bit word corresponding to a selectedpneumatic pressure reference,

pressure sensor means responsive to the pressure in said chamber forproviding an electrical output signal having a frequency which varies asa non-linear function of said chamber pressure, converter meansresponsive to said sensor signal for providing a further signal havingan amplitude proportional to the frequency of said sensor signal,

frequency-to-digital converter means for converting said sensorfrequency to a second multi-bit digital word corresponding to thepneumatic pressure within said chamber,

means responsive to said first and second multi-bit digital words forproviding a third multi-bit digital word corresponding to the differencetherebetween,

digital-to-analog converter means responsive to said difference word forproviding an analog error signal corresponding thereto,

means for supplying said further signal to said digitalto-analogconverter means whereby to vary the amplitude of the analog outputthereof as a function of said sensor frequency, and

means responsive to said analog error signal for positioning said valvemeans in a sense and to an amount to reduce said third digital word tozero said analog error signal for maintaining said valve position assaid error signal is reduced to zero, and

means responsive to said further signal proportional to sensor frequencyfor controlling the gain of said integration means.

1. A reference pneumatic pressure supply system, the combinationcomprising a reference pressure chamber, positionable valve meanscoupled with said chamber and a source of high pneumatic pressure and asource of low pneumatic pressure and adapted to control the resultant ofsaid pressures within said chamber, means for providing a firstmulti-bit digital word corresponding to a selected pneumatic pressurereference, pressure sensor means responsive to the pressure in saidchamber for providing a second multi-bit-digital word corresponding tothe pneumatic pressure within said chamber, means responsive to saiddigital words for providing a third multi-bit digital word correspondingto the difference therebetween, digital-to-analog converter meansresponsive to said difference word for providing an analog error signalcorresponding thereto, means responsive to said analog error signal forpositioning said valve means in a sense and to an amount to reduce saidthird digital word to zero, slew detector means responsive to the mostsignificant bits of said third multi-bit digital word for providing aconstant analog voltage output corresponding in sense to the sense ofsaid difference word, and means for supplying said constant voltage tosaid valve positioning means whereby to slew the pressure existing insaid reference pressure chamber rapidly toward said selected referencepressure.
 2. The reference pressure supply system as set forth in claim1 wherein said digital-to-analog converter means is responsive only tothe least significant bits of said digital difference word below themost significant bits thereof to which said slew detector means isresponsive.
 3. The reference pressure supply system as set forth inclaim 1 wherein said valve positioning means further includes integratormeans responsive to said analog error signal for maintaining said valveposition as said error signal is reduced to zero, said system furtherincluding means responsive to an output of said slew detector means forclamping said integrator.
 4. The reference pressure supply means as setforth in claim 3 further including, means responsive to the rate ofchange of the output of said slew detector for supplying a signal pulseupon the output of said slew detector going to zero and, means forsupplying said signal pulse to said integrator means to rapidly increasethe output of said integrator.
 5. A reference pressure supply systemcomprising the combination of a reference pressure chamber, positionablevalve means coupled with said chamber and a source of high pneumaticpressure and a source of low pneumatic pressure and adapted to controlthe resultant of said pressures within said chamber, means for providinga first multi-bit word corresponding to a selected pneumatic pressurereference, pressure sensor means responsive to the pressure in saidchamber for pRoviding an electrical output signal having a frequencywhich varies as a non-linear function of said chamber pressure,converter means responsive to said sensor signal for providing a furthersignal having an amplitude proportional to the frequency of said sensorsignal, frequency-to-digital converter means for converting said sensorfrequency to a second multi-bit digital word corresponding to thepneumatic pressure within said chamber, means responsive to said firstand second multi-bit digital words for providing a third multi-bitdigital word corresponding to the difference therebetween,digital-to-analog converter means responsive to said difference word forproviding an analog error signal corresponding thereto, means forsupplying said further signal to said digital-to-analog converter meanswhereby to vary the amplitude of the analog output thereof as a functionof said sensor frequency, and means responsive to said analog errorsignal for positioning said valve means in a sense and to an amount toreduce said third digital word to zero whereby to establish and maintainthe resultant pressure in said reference pressure chamber in accordancewith said selected pressure.
 6. The reference pressure supply system asset forth in claim 5 wherein said valve positioning means furtherincludes variable gain integrator means responsive to said analog errorsignal for maintaining said valve position as said error signal isreduced to zero, and means responsive to said further signalproportional to sensor frequency for controlling the gain of saidintegration means.